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MOTION OF A PARTICLE                       291
Case    I.   The orbit is a parabola, when v<? = 
'                            r0
2 k Case   II.   The orbit is an ellipse,   when v<?< 
2 k Case III.    The orbit is a hyperbola, when v<?> --
The general expression for the velocity, which is given by equation (9), may be put in the following special forms:
2k I.    v* =  ^ when the orbit is a parabola.
(2     ]\ --- ), when the orbit is an ellipse. r     ar
(2     1\ - + -), when the orbit is a hyperbola. /     a/
The quantity a is the length of the semi-transverse axis.
222. Velocity from Infinity.  The velocity which the particle acquires in falling towards the center from a point infinitely distant from the center is called the velocity from infinity. This velocity may be computed from the energy equation. Thus
^mv2= I F dr
r m ,
Therefore                     v*=-
But the last equation is identical with the relation which gives the velocity of a particle moving in a parabolic path, therefore if a particle describes a parabolic orbit its velocity at any point of its orbit is equal to the velocity it would have acquired if it had started from infinity and arrived at that